1. Field of the Invention
The present invention relates generally to the fabrication of stamps for microcontact printing using a master that contains surfaces of organic materials or composites of organic and inorganic materials, and more particularly a method of modifying the chemical nature of the surfaces of the master through a vapor phase treatment to improve the ease of separation of the stamp from the master.
2. Description of Related Art
Micromolding and microcontact printing as pattern transfer techniques are areas of significant interest for a wide range of applications, especially in the microelectronics industry. Microcontact printing, known as xe2x80x9cstampingxe2x80x9d, techniques are under development as low-cost alternatives to lithography for pattern definition on cost sensitive applications such as flat panel displays. Stamps made from siloxane polymers (such as poly(dimethylsiloxane) (PDMS)) are commonly used due to their ease of fabrication, conformal nature that allows stamping over topographical features, and potential for repeated use to pattern multiple substrates. Stamping has been demonstrated to transfer very thin coatings, down to monolayer coverage, of organics onto surfaces to act as etch barriers, plating seed layers, or ultra-thin patterns that can modify the interaction of the surface with subsequent coatings of organicsxe2x80x94either enhancing or resisting deposition of materials in specific areas. Imaging with siloxane stamps has been demonstrated down to submicron dimensions.
In molding and stamping applications, a curable (thermosetting) material, such as PDMS, or a pliable (thermoplastic) material such as a resist is formed into the desired shape by bringing it in contact with a rigid pre-formed master. Thermoplastic materials are generally cast from a solvent or heated until pliable and conformal with the master. Thermosetting materials are generally composed of monomers or polymer precursors that generally have low viscosity prior to curing (crosslinking), and conform easily to the surface of the master. Curing at room or elevated temperatures or under exposure to an appropriate source of radiation causes crosslinking of the polymer precursors, which will be transformed into an elastic or inelastic solid. To achieve an accurate replication of the master, the polymeric material must be in intimate contact with the surfaces of the master.
After the thermosetting or thermoplastic stamp has been formed to replicate the shape of the master, it must be separated from the master without causing damage to either the stamp or the master. The cured/formed polymer will have reduced flexibility, and may adhere to, or even be chemically bonded to the surface of the master. Adhesion of the stamp to the master increases the difficulty in separating the stamp from the master, and may result in fractures or tears in the polymeric stamp, with the torn sections of the stamp adhering to the master. Tear-outs result in stamps that are imperfect replicas of the master, and are unusable in manufacturing applications that require defect free reproductions. Tear-outs also reduce the utility of the master for production of future articles.
The master itself may be fabricated from a range of materials, although they are generally composed of inorganic materials, such as glass or silicon, or composite structures of an organic material on a glass or silicon substrate, although all organic masters are also feasible. Inorganic masters are generally produced by lithographically patterning a photoresist that has been spun onto the substrate. The exposed and developed photoresist creates a relief pattern that is the inverse of the desired stamp pattern. The patterns on the master may be as small as micron or sub-micron dimensions in width, but are usually several microns deep. Creating relief structures in a silicon or glass substrate requires transferring the resist pattern via wet chemical or plasma etching of the substrate. Such substrates can easily be treated with standard silane or fluorosilane solutions to increase the hydrophobicity of the surface and improve the release of organic materials molded from the master, as described in U.S. Pat. Nos. 5,425,848, 5,817,242 and 6,027,595. Release layers such as the silanes used in these patents are applied in dilute solutions of appropriate nonpolar organic solvents such has alkanes, chlorinated or fluorinated solvents, etc. These solvents provide adequate wetting and minimal interaction with inorganic masters, such as those made of silicon or glass.
A less time-consuming and more cost-effective technique for preparation of masters, also in common usage, is the fabrication of composite masters with a permanent patterned photoresist layer on a glass or silicon substrate. Using this fabrication technique, the exposed and developed resist images form the topographical features that will be replicated in the elastomeric stamp. No mention is made in the literature of treating such masters with release agents. The common solvents for silane release agents produce swelling or distortion of organic films, such as photoresists. Interactions with the organic features on the master limit the utility of solvent applied silanes to produce release layers on organic or composite organic/inorganic masters.
Other examples of the fabrication and uses of microcontact printing stamps and masters include U.S. Pat. No. 5,512,131, which describes the fabrication of elastomeric stamps and the use of these stamps to transfer self-assembled monolayers (xe2x80x9cSAMxe2x80x9d) of molecular species onto a solid substrate; and U.S. Pat. No. 5,900,160 describes the etching of said substrates after transfer of the SAM using an elastomeric stamp. U.S. Pat. No. 5,817,242 describes the use of a deformable layer as part of the stamp to accommodate for unevenness of the substrate being stamped. Fabrication of stamps in this patent includes transfer of resist features from one inorganic substrate to another.
U.S. Pat. Nos. 5,425,848 and 6,027,595 demonstrate the use of stamps to produce patterned resist images using molding techniques; U.S. Pat. No. 5,925,259 employs stamps to provide patterned xe2x80x9cmicrocontainersxe2x80x9d for etchants or reactants/catalysts to interact with the substrate in selected areas.
The present invention comprises exposing a composite organic/inorganic master to alkylchlorosilanes in the vapor phase. Chlorosilanes participate in facile reactions with hydroxyl groups existing on the surface of inorganic oxides (such as glass or the native oxides on silicon, aluminum, tin, etc.); or those in organics-containing phenolic or alcoholic groups, such as photoresists. The alkyl group on the silane can be chosen from a large selection of aliphatic or aromatic organic groups that have substituents with varying polarity and reactivity. The preferred materials to increase the hydrophobicity of the master and minimize adhesion of the stamp are fluorinated aliphatic chlorosilanes with at least eight carbon atoms, or long chain alkyl silanes with at least twelve carbon atoms, although aromatic silanes, such as phenyl silane are also applicable. To avoid swelling or distortion of the organic features on the master that can be caused by exposure to solvents, the chlorosilanes were brought into contact with the master in an evacuated, heated chamber, resulting in reaction of the silanes with all surfaces of the master to produce uniform, hydrophobic surfaces. The temperature of the reaction chamber affects the rate of reaction of the silane with the organic and inorganic surfaces of the master. Since the silanes chemically bond to the surfaces of the master, the hydrophobicity of the surface is retained for preparation of multiple stamps, increasing the useful lifetime (and so lowering the effective cost) of the master.